Robert W. Duemling, a career diplomat whose last assignment at the State Department was to head its Nicaraguan Humanitarian Assistance Office, has been appointed president and director of the National Building Museum.

The action was taken by the institution's board of trustees in late June, shortly before Duemling's official resignation from the State Department. The status of Bates Lowry, the museum's present director, is still under discussion.

The building museum, which opened in the fall of 1985, occupies the old Pension Building, a treasured 102-year-old brick structure near Judiciary Square. Although it has organized several excellent exhibitions and conducts research and educational programs, the museum's brief existence also has been troubled by financial and administrative uncertainties.

Duemling, 58, said yesterday that in accepting the job he is "returning to a first love." Both as an undergraduate and graduate student at Yale University (BS 1950, MA 1953) he specialized in the history of architecture and art. Under a Henry Fellowship to Cambridge University (1951-52) he studied with Sir Nikolaus Pevsner, a renowned architectural historian.

During his 30-year career at the State Department he served as ambassador to Suriname (1982-84), head of the Foreign Contingents Section of the Sinai Peacekeeping Force (1981-82) and deputy chief of mission at the American Embassy in Ottawa (1976-1980). He was director of the Nicaraguan Humanitarian Assistance Office from its inception in October 1985 until it was closed early this year.

Although he was subpoenaed as a witness to the congressional Iran-contra hearings, he was not called to testify. It was disclosed at the hearings and in earlier press reports that some of the same chartered planes that had delivered nonlethal supplies to the Nicaraguan rebels also had been used to deliver weapons.

"Those planes were available to anyone who wanted to pay the bill," Duemling said. "After they offloaded our {nonlethal} cargo, we no longer had anything to do with them ... Our role was to stick precisely to what the Congress had wanted and to protect it from inroads. The number one priority of my staff became to keep ourselves completely free of anything anybody else may have wanted to do."

"I was not forced out, I did not resign under any kind of cloud," he continued. "I began to look for a second career because I felt I have another solid decade and because there are not a lot of opportunities these days for career foreign service officers. There are too many political jobs at the top, is what has happened."

Some opposition to Duemling's appointment was expressed in meetings of the museum's board, according to Chairman Herbert M. Franklin. "It wasn't to him personally," Franklin said. "It was about wanting to give the job to somebody with a museum background." The appointment was approved with one negative vote, he said.

The museum had been operating with a "twin tower" organization, Franklin said, meaning that there were two executives, a president and a director, the one with administrative and financial responsibilities, the other primarily in charge of exhibitions and education -- "the artistic side of things."

After Robert McKean resigned as president last November, trustee Charles Horsky stepped in to do the job, pro bono, on an acting basis with Lowry continuing as director. "We found it didn't work very well and felt we needed to combine the jobs in the person of a true chief executive officer," Franklin said. "Basically it came down to your perception of the abilities of one human being." The board was impressed, he said, with Duemling's management experience and his "special affinity for and understanding of the museum and its programs."

Lowry's position in the new organization remains unresolved. An architectural historian, former director of the Museum of Modern Art in New York and former chairman of three different university art departments, Lowry has been director of the museum since 1980. "We're undertaking to find a relationship with Bates that would work and we've proposed some things and he has made proposals," Franklin said. Discussions have been put on hold because Lowry and his wife Isabel, also an architectural scholar, are recuperating in Upstate New York from injuries suffered in an automobile accident last month. Lowry could not be reached for comment.

Among the principal uncertainties the museum has faced in its early years has been on-again, off-again support in Congress. The 1980 legislation making the Pension Building available to the museum also authorized $500,000 annually in operating funds for programs relating to historic preservation. None of that money was appropriated, although the federal government does maintain the building and is providing approximately $200,000 of the museum's $1.2 million operating budget this fiscal year. The rest of the money was raised privately from the museum's wide-ranging constituency of architects, builders and construction trade unions.

Renovation of the building also has proceeded at an uneven pace. The museum was closed to the public last month after a major, federally financed renovation picked up speed. Ground-floor galleries bordering the huge Great Hall -- one of Washington's truly spectacular spaces -- will reopen in October, according to a museum spokesperson, but the renovation will not be completed until late next year or early 1989.

"This museum is now maturing after a period of infancy," Franklin said. He identified Duemling's chief tasks as negotiating with the federal government over the "nitty-gritty" details of the renovation, developing exhibitions and acquisitions policies, and increasing financial and other support from the private sector. "Now that we've begun to demonstrate the possibilities," Franklin said, "we need to raise {more} unrestricted operating funds.